1. Field of the Invention
The present invention relates to a coating apparatus and a coating method. More particularly, the invention relates to a coating apparatus and a coating method, which can coat a web stably with a coating liquid by a bar coater even when the web travels at a high speed.
The invention further relates to a coating apparatus and a coating method, which can be suitably used for forming a coating film of a laminated structure having two or more layers, over a band-shaped object to be coated.
The invention further relates to a rod coating apparatus and, more particularly, to a rod coating apparatus and a rod coating method for coating a web such as a band-shaped metal sheet with a coating liquid by transferring it using a coating rod.
The invention further relates to a coating apparatus and a coating method which can continuously coat stably for a long time when a plurality of layers are to be formed by coating a long band-shaped substrate stably and continuously for a long time with two or more coating liquids.
2. Description of the Related Art
A planographic printing plate is usually manufactured by graining at least one face of a web of pure aluminum or aluminum alloy, by forming an anodized film, if necessary, on that face to form a support web, and by coating the grained face of the support web with a photosensitive layer forming liquid or a heat-sensitive layer forming liquid and drying it to form a photosensitive or a heat-sensitive engraving face.
In order to coat a band-shaped web such as the support web with a coating liquid such as a photosensitive layer forming liquid or a heat-sensitive layer forming liquid, it is customary to use a bar coater.
The bar coater may be constructed to have a compact coating head, and the bar coater is widely used for applying the photosensitive layer forming liquid and the heat-sensitive layer forming liquid.
The bar coater of the prior art generally has a bar and a coating portion mounted thereon. The bar rotates in the same or opposite direction as or to the traveling direction of the web while touching the lower face of a continuously traveling web. The coating portion discharges, at the traveling time of the web, a coating liquid to the upstream side of the bar with respect to the traveling direction of the web to form a coating liquid pool thereby to coat the lower face of the web with the coating liquid. In the following, the upstream side or the downstream side with respect to the traveling direction of the web will be simply referred to as the “upstream” or the “downstream”.
As the bar coater, moreover, there is an SLB type bar coater (Japanese Utility Model No. 2054836). This SLB type coater is provided with a first weir plate which is disposed upstream of and adjacent to the bar and formed to have a smaller thickness downstream with respect to the traveling direction of the web. The first weir plate is bent at its upper end portion toward the bar and has a flat face of a length from 0.1 to 1 mm at its top portion. There is another PBS type bar coater (Japanese Patent Application Publication No. 58-004589). This PBS bar coater is provided with a first weir plate formed to have a smaller thickness downstream at its upper end portion, a bar and a second weir plate disposed downstream of the bar.
When the traveling speed of the support web is raised, however, the air to run following the support web, i.e., an entrained air film or a film of entrained air is formed over the surface of the support web.
In either of the SLB bar coater or the PBS bar coater, the entrained air film, if formed over the surface of the support web, is carried by the support web to a coating liquid pool in the coating portion. In the coated film of the coating liquid formed on the surface of the support web, moreover, a film runout, a ripple streak and/or a coating heterogeneity occurs so that the coating of the coating liquid cannot be stabilized.
Next, the printing plate layer may have a multi-layered structure having two or more layers. Over the surface of the printing plate layer, moreover, an anti-oxidant layer of polyvinyl alcohol may be formed.
A bar coater is used when a printing plate layer forming liquid is applied to form the printing plate layer having the multi-layered structure. When second and subsequent layers are to be formed, the bar of the bar coater may come into contact with the first layer to scrape off and damage the first layer, or the coating film of the first layer scraped off by the bar may migrate into the printing plate layer forming liquid returned from a return line in the bar coater thereby to contaminate the same. During forming the first layer, moreover, the bar may touch the grained face of the support web thereby to damage the grained face.
Next, the planographic printing plate is manufactured by forming a visible exposure type or a laser exposure type printing plate layer on the roughed surface of the support web anodized, if necessary. Moreover, the printing plate layer is usually formed by coating the roughed surface of the support web with either a photosensitive layer forming liquid containing a photosensitive resin or a heat-sensitive layer forming liquid containing a thermally polymerizable resin and by drying the support web.
In the recent years, there has been studied the planographic printing plate which has a printing plate layer of a multi-layered structure having two or more layers. This planographic printing plate can be exemplified by the visible light exposure type planographic printing plate having two or more photosensitive layers or the laser exposure type planographic printing plate with a heat-sensitive layer as the first layer and a photo-thermal conversion layer as the second layer.
When all the layers of the multi-layered printing plate layer are formed by the bar coater, the bar of the bar coater may come into contact with the first layer to scrape off and damage the first layer when the second and subsequent layers are to be formed. Moreover, the coating film of the first layer scraped off by the bar may migrate and contaminate the photosensitive layer forming liquid or the heat-sensitive layer forming liquid returned from the return line of the bar coater. Thus, it has been desired that the planographic printing plate having the printing plate layer of the laminated structure can be stably formed by using the bar coater.
Next, as a rod coating apparatus for coating a web face with a coating liquid, there is a method for coating the web with a coating liquid at a coating amount adjusted with a coating rod, by bringing the coating rod contact with the web. This rod coating method is widely used because it is featured by forming a thin coating layer with an extremely simple construction and at a high speed.
This rod coating method is as disclosed, for example, in Japanese Patent Application Laid-Open (JP-A) No. 53-22543 or JP-A No. 59-123568. Of this method, the coating rod (as will be shortly referred to as the “rod”) is brought into contact with the traveling web, and a coating liquid pool is formed just before the contact portion of the web and the rod. The coating liquid in the pool is scooped up and transferred to the web by the rod forced to rotate in the traveling direction of the web.
There still remains a defect that the web is scrubbed unless the rotating speed of the rod and the traveling speed of the web are well synchronized although the rod is forced to rotate into contact with the web.
Therefore, a coating apparatus was proposed as referred to JP-A 12-107665. In the apparatus, the rod is forced to rotate at a rotating speed of 10% or higher of that at the coating time or at the following time and in which the transmission of the forced rotating force is interrupted after the rod made contact with the web, to cause the rod to follow the web and to rotate.
In the practice, however, it takes a constant time to complete the transmission interruption actually after a command to release the interruption of the rotating force was issued from a controller. Therefore, it takes a long time for the rod to start to touch and follow the web. As a result, the time period, for which the traveling speed of the web and the rotating speed of the rod are not synchronized, is elongated to restrict the improvement in the production efficiency.
If the resistance to the rod rotation is raised by disturbances and the rod stops its rotation, the web will be continuously scratched.
Next, in the manufacture of the planographic printing plate, there is included a procedure to form an anti-oxidant layer on the surface of the printing plate layer. In the planographic printing plate having multiple printing plate layers or the planographic printing plate having the anti-oxidant layer over the printing plate layer, it is cursory to form every layers one by one. These individual layers are usually formed by applying the coating liquids such as a photosensitive layer forming liquid containing a photosensitive resin, a heat-sensitive layer forming liquid containing a thermally polymerizable resin and an anti-oxidant layer forming liquid composed mainly of an aqueous solution of polyvinyl alcohol, and then by drying the applied liquids.
In the case where the photosensitive layer is formed by coating the support web with the photosensitive layer forming liquid and then by drying it and where the anti-oxidant layer is then formed by coating the photosensitive layer with the anti-oxidant layer forming liquid and then by drying it, an uncoated portion will be left uncoated with the photosensitive layer forming liquid for some reason. At this time, the anodized layer of the support web is exposed at the uncoated portion. The anti-oxidant forming liquid has an excellent wettability to the photosensitive layer but not always to the anodized surface film.
Despite of this wettability, the coated layer of the prior art is continuously coated with the anti-oxidant forming liquid even with the uncoated portion. In this uncoated portion, therefore, a coating defect of the anti-oxidant layer occurs to raise a sixth problem that the anti-oxidant layer forming liquid is not homogeneously applied. Even if the coating defect in the photosensitive layer forming liquid is corrected and the photosensitive layer forming liquid is normally applied, coating defect may not be eliminated once it occurs.